Management of an energy storage system, such as an electric battery or an ultracapacitor or supercapacitor energy storage system, is essential to ensuring long life, efficiency, and reliability of the energy storage system and an equipment powered by the energy storage system. Proper management requires real time knowledge of cell voltage, i.e. voltage at each energy storage cell should be permanently monitored. Energy storage systems include multiple energy storage cells connected in series, i.e. “stacked”, so that individual cells near the top of the stack may be at elevated voltages with respect to the system ground.
FIG. 1 illustrates a battery 10 representing an example of an energy storage system. The battery 10 is composed of multiple cells connected in series. While a cell arranged closer to the ground terminal is at a low potential, another cell connected at the other side of the battery stack may be at a high potential. For example, eighty-one 4.2V cells shown in FIG. 1 would cause a total voltage of approximately 340V. Therefore, voltage measuring devices capable of measuring voltages of individual cells must withstand very high voltages. Using such high voltage measuring devices for stacked battery management would be extremely expensive and prone to errors.
Another example of an energy storage system is an ultracapacitor or supercapacitor system. Ultracapacitors or supercapacitors represent one of the latest innovations in the field of electrical energy storage, and find their place in many applications involving mass energy storage, power distribution. They are of particular interest in automotive applications for hybrid vehicles and as supplementary storage for battery electric vehicles. In comparison with classical capacitors, these new components allow a much higher energy density, together with a higher power density. Ultracapacitors or supercapacitors may be produced based on a double-layer capacitor technology to increase their charge density. However, double layer capacitors have a relatively low maximum voltage. This necessitates a series connection of cells to support operation at higher voltages in order to reach an acceptable power conversion efficiency.
The higher the cell voltage, the shorter the expected life of a double layer capacitor. Therefore, cell voltages in the ultaracapacitor or supercapacitor system should be monitored to prevent voltages at individual cells from exceeding maximum values. Also, ultracapacitors or supercapacitors must be monitored during charging to prevent the charging voltage from exceeding the rated voltage.
A common method of measuring voltages at high common modes involves a 4-resistor difference amplifier having a resistor network arranged as a common-mode voltage divider. For example, this arrangement is used in the LT®1990 difference amplifier developed by the Linear Technology Corporation, the assignee of the present application. However, matching resistors in the resistor network is a difficult problem. Mismatched resistors may compromise measurement accuracy as the common mode voltage increases. Also, the resistor dividers represent a load on the battery.
Another known method of measuring voltage involves capacitive switching. This method is used for measuring a low voltage, for example, in the LTC®1043 dual switched capacitor building block developed by the Linear Technology Corporation, the assignee of the present application. In this block, a pair of switches alternately connects an external capacitor to an input voltage and then connects the charged capacitor across an output port. However, at high common mode voltages, this method would require high voltage MOSFETs, which are not readily available in monolithic chips.
A further known method includes direct digitization, and level shifting of digital information. This type of measurement is described in Linear Technology Design Note DN341 by Mark Thoren entitled “16-bit ADC Simplifies Current Measurements,” although the Design Note relates to measurement of current rather than voltage. The Design Note describes a −48V telecom supply current monitor using 16-bit Delta-Sigma analog/digital converter (ADC) for direct current (DC) measurements. The monitor uses optoisolators as level shifting devices for data transfer. However, this method is appropriate for a small number of measurements. Measuring voltage at a large number of cells would require many optoisolators (or transistors) and become cumbersome.
Therefore, it would be desirable to create a simple and efficient technique for monitoring voltages at multiple energy storage cells connected in series.